1. Field of the Invention
The invention concerns a method of vitrifying a porous cylindrical article made out of glass soot, especially for manufacturing a preliminary blank for optical fibers and a furnace for carrying out the method. In this method the article is heat-treated in a furnace in a vacuum or in an atmosphere that contains helium. Glass soot consists of fine vitreous particles of essentially silicon dioxide that are derived from the gas phase, manufactured, for example, by flame hydrolysis from silicon tetrachloride, and are precipitated onto a carrier, whereby an additive, germanium dioxide for instance, that alters the index of refraction of the precipitate can optionally be included during the precipitation.
2. Background Information
Methods of the aforesaid type are known, from German Pat. No. 2,827,303 for example. The porous glass-soot article in the form of a hollow cylinder is compacted by suspending it vertically from a tubular support and inserting it at a prescribed rate into a heated section of a furnace from above, so that the article, commencing at its free end and terminating at its suspended end, becomes progressively compact. The heat treatment can be carried out in various known inert-gas atmospheres, for example, helium, nitrogen, argon, or mixtures thereof. The channel that extends axially through the article can be connected to a vacuum pump.
In the known method, the original cross-section of the article that is to be compacted into a dense piece of glass is retained, with a particular section of the article being non-circular, elliptical, for example, and that shape is maintained during the heat treatment. The same is true when the cross-section is circular and increases or decreases along the axis.